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ISSN: 1511-3701 Pertanika J. Trop. Agric. Sci. 33 (2): 243 - 250 (2010) © Universiti Putra Malaysia Press

Dormancy and Cardinal Temperatures during Seed Germination of Five Weedy Rice (Oryza spp.) Strains

Adam B. Puteh*, Roziah Rosli and Rosli B. Mohamad Department of Crop Science, Faculty of Agriculture, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia *E-mail: [email protected]

ABSTRACT Temperature during seed imbibition has been found to influencegermination rate and final percentgermination. Seeds of one cultivated variety and five weedy rice strains, collected from different localities in Peninsular Malaysia, were used to determine their degree of dormancy and cardinal temperatures. Meanwhile, standard germination and tetrazolium chloride (TTC) tests were used to evaluate the percentage of seed viability and degree of dormancy. Seed germination test at six different constant temperatures (between 10 and 35oC) was

applied to determine the cardinal temperatures estimated by linear regression models, base temperature, Tb,

optimum temperature, To, and maximum temperature, Tc. The TTC test was found to be a simple and quick test to determine the degree of seed dormancy among different weedy rice strains, when used together with a standard germination test. Germination rate was found to be related to the degree of dormancy but it had no

influence on the range ofcardinal temperatures. The Tb among the fiveweedy rice strains was in the range of o o o 2-7.3 C. The To varied between 28.1 and 37.5 C, with an average of 32.5 C. This temperature (To) was higher o o than that of the cultivated MR73 variety (24.3 C), whereas the range of Tc was 42.2-43.3 C. The study indicated

that the non-dormant cultivated rice seed had lower Tb and To values than the dormant seed of weedy rice.

Keywords: Weedy rice, germination, seed dormancy, cardinal temperatures

INTRODUCTION the temperature range at which weedy rice Weedy rice (Oryza spp.) infestation occurs in seed germinates will help to predict seedling most major rice-growing areas of the world. emergence. The invasive nature of weedy rice has become Temperature is an important single factor a major concern, especially in direct seeded rice affecting the capacity for germination by cultivation. In Malaysia, the presence of weedy regulating dormancy, and it also critically rice in a direct-seeded field can reduce yield up determines the rate of progress toward to 74% (Bakar et al., 2000). Early shattering completion of germination once a seed is has enabled them to escape harvest, while seed stimulated (Alvarado and Bradford, 2002; dormancy ensures their survival in the soil seed Bradford, 2002). The degree of seed dormancy bank. The degree of infestation through soil seed influences the temperature range, at which seed bank germination varies between production will germinate, with the range increasing as years. Nonetheless, the reasons behind these seeds loose dormancy (Benech-Arnold et al., observations are not known. Thus, determining 2000; Vegis, 1964). These critical temperatures,

Received: 3 April 2009 Accepted: 8 December 2009 *Corresponding Author Adam B. Puteh, Roziah Rosli and Rosli B. Mohamad which are commonly referred to as cardinal temperature during imbibition, and there has temperatures, consist of the base and maximum been no record of its cardinal temperatures temperatures, below or above which germination to date. Thus, understanding the variation will not occur, while the optimum temperature is in temperature during seed imbibition may where germination is the most rapid (Bradford, establish germination responses of weedy rice 2002). This concept was initially proposed on strains seedling emergence in the field. The the whole plant basis, but it is also applicable aims of this study were to (1) determine the to seed during germination. Bewley and Black cardinal temperatures, base (Tb), optimum (To)

(1994) described cardinal temperatures as and maximum (Tc) of the different weedy rice the range of temperatures over which seeds strains, and (2) determine the germination rate of a particular genotype could germinate. In within these cardinal temperatures. cultivated rice varieties, a high percentage of germination is attained in two days at 27-37oC, MATERIALS AND METHODS while no germination is found to occur at 8oC and 45oC (Yoshida, 1981). However, the cardinal Plant Material temperatures for seed germination have never For the purpose of this study, five weedy rice been reported in weedy rice strains, which strains and one cultivated variety were used. are helpful for predicting the degree of seed Seeds of the cultivated variety MR 73 were dormancy and infestation in the field. obtained from the Malaysian Agriculture Models which describe seed germination Research and Development Institute (MARDI). behaviour, in response to a range of temperatures The seeds of five weedy rice strains were during imbibition, have been proposed and randomly collected from several locations developed (Covell et al., 1986; Ellis and in Malaysia, namely Seberang Perak, Kuala Butcher, 1988; Alvarado and Bradford, 2002; Pilah, Besut, Perlis, and Kemubu Agricultural Hardegree, 2006). The model commonly Development Authority in Peninsular Malaysia. known as the thermal time model has been Hereafter, these five strains are termed as SP, KP, extensively used and successfully applied Besut, Perlis and KADA strains, respectively. to describe germination timing and seedling The seeds were collected in January and emergence in crops (Finch-Savage and Phelps, February 2008 and stored at 0oC in double sealed 1993) and weed species (Roman et al., 2000). plastic bags for two months before conducting This particular thermal time model predicts the study. Seed moisture was kept in the range germination rate at sub-optimal temperatures of 9-11% prior to storage and at the start of the (from the minimum temperature to the optimum experiments. temperature) and supra-optimal temperature (i.e. from the optimum temperature to the maximum temperature) in a linear function (Hardegree, Seed Viability Tests 2006). The thermal time model has also been For standard germination test, the imbibing seeds successfully used to predict weeds seedling were left at ambient temperature in the laboratory emergence in the field under temperate growing at 25±3oC. The seeds were germinated on double conditions (Forcella et al., 2000; Vleeshouwers layered moistened (±10mL distilled water) filter and Kropff, 2000). papers (Whatman, no. 1, in 80 mm diameter by Natural selection on germination responses 10 mm deep disposable plastic Petri dishes. The to seasonal environmental cues in some seeds were soaked in 10% Clorox for surface species has been proposed as a significant sterilization for 3 min. prior to the testing. determinant for the genotype to establish in a Seedling evaluation on the number of given seasonal environment (Donohue, 2005). germinated seeds was done daily, starting on day However, little information is available on the 2 after imbibition for 14 days. Seeds which did germination responses of weedy rice strains to not germinate after 14 days were considered as

244 Pertanika J. Trop. Agric. Sci. Vol. 33 (2) 2010 Dormancy and Cardinal Temperatures during Seed Germination of Five Weedy Rice (Oryza spp.) Strains dead seeds. In contrast, seeds were considered The reciprocals of the time to germination were as germinated when the radical emergence was plotted to estimate the optimum temperature, at >5 mm. The test was replicated twice with 50 which the rate of germination was maximum seeds per replication. (To). The rates of germination were also For the tetrazolium chloride test, two subjected to the linear regression analysis to replicates of 50 seeds per replicate were imbibed describe cumulative germination response in distilled water at room temperature for 24 h. of temperature (SAS Institute, 2005). The The middle portion of the seeds was pierced cumulative percentage germination (CGP), with a needle before soaking them in 0.1% of obtained from the germination tests at different 2, 3, 5-triphenyl tetrazolium chloride (TTC) temperatures, were used to calculate the cardinal salt solution. The seeds imbibing in the TTC temperatures. Intersected-line models were solution were exposed to 35oC in the oven for used as proposed by Garcia-Huidobro et al. 2 h. Viable seed was evaluated based on the (1982). The equation used to describe the rates topographical staining pattern on the embryo, as of germination between base and up to optimum described in the ISTA procedure (ISTA, 1993). temperatures is as follows:

1/t = (T – Tb)/θ1 (1) Seed Germination Test at Different Temperature In order to describe the germination responses A germination test at different temperatures for above To, but below the maximum temperature all the seeds was conducted in the germination (Tc), equation (2) was used: chamber in darkness. Nonetheless, the 1/t = (Tc – T)/θ2 (2) preliminary works did not show any seed sensitivity to light during germination (Rosli, where t is the time taken in days for the CGP 2008). Meanwhile, the preparation of seeds and to reach a given percentage, T is the temperature, imbibition media are similar to the procedures while Tb, To and Tc are the base, optimum and used for the standard germination test, as maximum temperatures, respectively. These described above. The imbibing seeds were models predict the germination rate for a given exposed to the constant temperatures of 10, seed fraction (sub-optimal and supra-optimal 15, 20, 25, 30, or 35oC. This experiment was range) in a linear function of temperature. The performed in two replications consisting of 50 intercepts of the fitted linear regression lines on seeds per replicate. Seeds were considered as the temperature axes were used to estimate Tb germinated when the radicle was >5 mm. The and To. To, was calculated as the intercept of sub- evaluation was done daily for 20 days, beginning optimal and supra-optimal temperature function on Day 2 after sowing. (Hardegree, 2006).

Statistical and Data Analysis RESULTS All the collected data were subjected to the Viability and Degree of Dormancy analysis of variance using the Statistical Analysis Based on the standard germination test, the System (SAS) Software, version 8.2. When initial quality of the five weedy strains was in ANOVA indicated a significant effect, the least the range of 19-86% (Table 1). The germination significant difference (LSD) was performed to percentage of the cultivated variety (MR73) was determine significant differences among the found to be the highest (98%). Meanwhile, the means of the treatments. lowest germination percentage among all the Meanwhile, the germination rates were weedy rice strains was observed in the KADA calculated as the inverses of times to radicle strain and the highest was in the SP strain, with emergence (Alvarado and Bradford, 2002). 19% and 86%, respectively.

Pertanika J. Trop. Agric. Sci. Vol. 33 (2) 2010 245 Adam B. Puteh, Roziah Rosli and Rosli B. Mohamad

The TTC test used was to determine the the SP strain, as compared to the percentage of percentage viability, as well as the degree of viability based on the standard germination test. dormancy among the weedy strains and for MR Based on the TTC test, the viability percentage 73. The highest percentage of viability was also was found to be 70-83%. Based on this test, the recorded in MR73 with 99% (Table 1). This seed of SP strain was not dormant. As for Besut suggests that no seed dormancy is present in this and Perlis strains, the seeds appeared to have a particular cultivated variety. Based on the TTC slight dormancy. The data indicate that there is a test, a higher percentage of seed viability was variation in the degree of seed dormancy among observed in all the weedy rice strains, except for the weedy strains used in this study.

TABLE 1 Percentage viability of the cultivated rice variety and weedy rice strains based on the standard germination and tetrazolium chloride (TTC) tests

Strain/Variety Cumulative germination (%) TTC (%)

MR 73† 98 99 Seberang Perak 86 83 Kuala Pilah 51 76 KADA 19 74 Besut 65 71 Perlis 74 80 LSD 10 2

† cultivated rice variety

TABLE 2 The germination percentage of the cultivated rice variety and weedy rice strains at constant temperatures

Percentage germination Variety/strain 10˚C 15˚C 20˚C 25˚C 30˚C 35˚C

MR 73† 58 78 94 98 94 29

Seberang Perak 8 10 54 62 82 54

Kuala Pilah 20 32 50 54 60 34

KADA 0 0 0 8 22 12

Besut 0 0 34 28 48 22

Perlis 6 8 20 24 50 36

LSD 26.2 16.6 17.0 14.1 20.2 19.6

† cultivated rice variety

246 Pertanika J. Trop. Agric. Sci. Vol. 33 (2) 2010 Dormancy and Cardinal Temperatures during Seed Germination of Five Weedy Rice (Oryza spp.) Strains

Germination and Germination Rate at temperature to >30oC was found to cause a Constant Temperatures rapid decline in the germination percentage The increase in the temperature (i.e. from of both the cultivated variety and weedy 10oC to 30oC) during imbibition enhanced the strains. Meanwhile, the maximum germination germination percentage of both the cultivated percentage of the cultivated variety was observed o o rice variety and weedy strains (Table 2). at 25 C, whereas this was observed at 30 C for All the weedy rice strains were found to all the weedy rice strains. No germination was have low germination percentages (≤82%) as observed in KADA weedy strain at temperature o compared to MR73 (>90%) within the range of 20 C and below, indicating that this strain is constant temperatures. Increasing the imbibition highly dormant.

10 20 30 40 50 ) -1

0 10 20 30 40 50 Germination rate (d

Temperature (oC)

Fig. 1: Germination rates at the suboptimal and supraoptimal temperature range in response to the different temperatures for MR73 cultivated rice variety, Seberang Perak,

Kuala Pilah, KADA, Besut and Perlis weedy rice strains. Tb, To, and Tc indicate base, optimum and maximum temperature, respectively. * and ** , significant at P<0.05 and <0.01, respectively

Pertanika J. Trop. Agric. Sci. Vol. 33 (2) 2010 247 Adam B. Puteh, Roziah Rosli and Rosli B. Mohamad

The higher germination percentage in was observed in the KADA strain (0.014 day-1), the cultivated variety at different constant while the highest was in SP strain with 0.128 temperatures (10-30oC) could be attributed to day-1, based on the linear regression model in the the relatively higher germination rate (Table sub-optimal range. 3). Similarly, a lower germination percentage The decline in the germination rate within in weedy rice strains was due to the lower the supra-optimal range for the weedy rice germination rate, particularly at 25oC and lower. strains was between -0.078 day-1 to -0.33 day- The highest germination rate in the cultivated 1. Meanwhile, the cultivated rice variety and variety and the weedy strains was observed at SP weedy strains had similar germination rate 25 oC and 30 oC, respectively. (within the supra-optimal range of temperature), suggesting that the weedy rice strain has a similar germination characteristic with the cultivated Germination Rate and Cardinal Temperatures rice variety (MR73) at higher temperature. Based on the Linear Model Within this supra-optimal range of temperature, The estimated germination rates, within the sub- the KADA weedy strain was found to have the optimal and supra-optimal range of temperatures, lowest estimated germination rate of -0.078 vary between the weedy rice strains. All the day-1. germination rates, which were calculated from The germination rate for the weedy rice the estimated germination time course, showed strains and the cultivated variety increased a significant correlation with temperature at linearly with the increase in the germination both the sub-optimal and supra-optimal ranges temperature (Fig. 1). Meanwhile, the lowest o of temperatures (Fig. 1). The highest estimated estimated Tb for MR73 was 0.4 C (Table 4 and germination rate was recorded for MR73, which Fig. 1). The range of the estimated Tb for the was 0.226 day-1 in the sub-optimal range. On the weedy rice strains was between 2.0 - 7.3oC, while contrary, the lowest estimated germination rate the KP strain had the lowest Tb. The To for the

TABLE 3 The germination rate at different imbibition temperatures of a cultivated rice variety and weedy rice strains

Germination rate (d - 1 ) Variety/strain 10˚C 15˚C 20˚C 25˚C 30˚C 35˚C

MR 73† 2.9 3.9 4.7 4.9 4.7 1.5 Seberang Perak 0.4 0.5 2.7 3.1 4.1 2.7 Kuala Pilah 1.0 1.6 2.5 2.7 3.0 3.0 KADA 0 0 0 0.4 1.1 0.6 Besut 0 0 1.7 1.4 2.4 1.7 Perlis 0.3 0.4 1.0 1.2 2.5 2.8 LSD 1.3 0.8 0.9 0.7 1.1 1.0

† cultivated rice variety

248 Pertanika J. Trop. Agric. Sci. Vol. 33 (2) 2010 Dormancy and Cardinal Temperatures during Seed Germination of Five Weedy Rice (Oryza spp.) Strains

o seed germination ranged from 28.1 to 37.5 C for temperatures. The seed of the KADA strain was the weedy rice strains (Table 4). The estimated found to be very dormant and thus had the lowest o To for MR73 was found to be the lowest (24.3 C) germination rate. However, the range of the as compared to the weedy rice strains, whereas cardinal temperatures, Tb to Tc, was quite similar o the highest To of 37.5 C was observed in the to the non-dormant seed of the SP strain. The

KADA strain. Nonetheless, Tc did not differ results indicated that for the non-dormant seed much between the weedy rice strains and MR73. (e.g. in cultivated MR73 variety), Tb would be

The narrow range of Tc among the two varieties shifted to a much lower temperature relative to o was between 42.2 – 43.3 C, suggesting that the the weedy strains. Meanwhile, To for the weedy non-dormant rice seed will not germinate above strains, except for the KP strain, was above 30oC. 43oC. In this study, the increase in the degree of seed

dormancy does not shift Tc in rice, but it changes T to a much higher temperature. DISCUSSION o Tc was almost similar among the weedy rice Under ideal germination environments, such as strains and the cultivated variety. This suggests in the laboratory condition, seed dormancy is that the maximum temperature limit for seed strongly imposed in some weedy rice strains. germination is species specific, and it is not Weedy rice strains have often been associated influenced by seed dormancy. It is interesting with seed dormancy (Gu et al., 2005). The to note that the degree of seed dormancy results indicated that the seed of the KADA will change, i.e. either shorten or widen, the strain is highly dormant relative to the seeds of sub-optimal and supra-optimal ranges of other strains. However, the data presented in temperature. This study has clearly indicated this study are still insufficient to determine the that the sub-optimal range of the temperature type or class of dormancy involved in the tested of the dormant seed is widened by 26-31oC weedy rice strains. Reducing the percentage of temperature points compared to the non-dormant viability between the TTC test and the standard cultivated variety and the SP strain. The supra- germination test will indicate the degree of optimal range of temperature is concurrently dormancy of a seed lot. The SP strain does shortened with the increase in seed dormancy. not have seed dormancy, suggesting that not The results also provide evidence that there is all weedy rice strains have seed dormancy. It an ecotypic variation in the base and optimum appears that the strain of SP is closely related to temperatures for seed germination in weedy rice. the cultivated variety. Therefore, those weedy Fluctuation in soil temperatures is rice strains producing non-dormant seeds will commonly associated with weed seed emergence result in a more widespread infestation in the in several species (Foncella et al., 2000; field throughout the year. Vleeshouwers and Kropff, 2000). However, in The degree of seed dormancy in weedy rice tropical growing environments, fluctuation in varies between the strains. Since the strains soil temperature may be negligible, yet higher used in this study were collected from different weedy rice infestation is commonly observed locations, environmental conditions are therefore when temperature during preceding growing suggested to play important roles in determining season is higher. Higher To requirement for the the degree of dormancy. The variation in the weedy rice strains observed in this study could degree of seed dormancy is not only influenced possibly trigger the germination process and the by the environment at the location where the reason for high weedy rice infestation when the plants are grown, but it is also influenced by temperature during preceding growing season genetic factors (Li and Foley, 1997; Gu et al., was above normal. 2005). The rate of germination appears to be related to the degree of dormancy, but it does not seem to be related to the range of cardinal

Pertanika J. Trop. Agric. Sci. Vol. 33 (2) 2010 249 Adam B. Puteh, Roziah Rosli and Rosli B. Mohamad

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